Applications that require manipulation of many optical signals are becoming more complex and more commonplace. Such applications include the routing of signals in fiber optic networks, necessitated by, for example, telecommunications and large volumes of internet traffic. In fiber optic networks, large volumes of optical signals must be transmitted between optical fibers and optoelectronic devices; optical signals must be processed and retransmitted as electrical or optical signals; electrical signals must be processed and transmitted as optical signals. The optical fibers are typically present as arrays of up to 100×100 fibers; larger arrays are possible and may be expected in the near future. As optical interconnection networks become more complex and the volume of signal traffic increases, it becomes more and more important to reduce signal loss and cross-talk and to minimize the size of the optical/optoelectronic interconnect package. It would be desirable to intimately integrate electrical and optical signal processing, transmission and reception, in a single robust unit providing efficient optical coupling to optical fibers.
The optoelectronic devices that perform the optical signal processing tasks, such as lasers and photodetectors, are typically made in various optically active materials, such as compound semiconductors, most commonly III-V materials, especially GaAs, as well as II-VI semiconductors such as ZnSe, transparent ferroelectrics such as lithium niobate and other related oxide materials, and liquid crystal and other optoelectronic polymers. Optoelectronic devices may be fabricated in epitaxial layers grown on suitable substrates which are not ordinarily silicon. For example, vertical cavity surface emitting lasers (VCSELs) are typically fabricated in AlGaAs and GaAs on GaAs substrates. Optoelectronic devices may also be fabricated in single crystal materials such as oxides.
The electronic control circuitry for the optical/optoelectronic interconnect systems is typically formed in silicon. Silicon-based metal-oxide semiconductor (MOS) technology is virtually the standard for digital circuits that are used for to control the signal processing tasks in switching systems. Increasingly, CMOS (complementary MOS) technology is utilized in these applications. CMOS technology incorporates both n-channel MOS and p-channel MOS transistors in the same monolithic structure. No other approach can compare with the high device densities and high yields available with silicon CMOS technology.
It is necessary to find methods to combine high density, high speed CMOS circuitry with optoelectronic devices in an intimate fashion in order to minimize parasitic capacitance and inductance and to increase density of optical interconnects. Applications for such a capability include formation of transmitter, receiver or amplifier modules for optical fiber communication. Integrating the CMOS circuitry with the optoelectronic devices has the potential to greatly increase the efficiency of the overall optical/optoelectronic signal processing package.
In high speed telecommunications and data networks, such CMOS circuitry-controlled optoelectronic devices must efficiently transmit and receive optical signals to and from optical fibers. Optical fibers have an optical core that transmits light and an outer cladding layer that has a lower refractive index than the optical core. Optical fibers used in networks may be multi-mode or single mode, depending on the size of the optical core, and selected based on the distance over which the optical signal needs to be transmitted and the bandwidth desired. Optical fibers are formed from a variety of materials such as glasses and plastics, glasses being predominant. It is understood that the optical signals are transmitted in the optical core along the fiber's long axis, and at the fiber's end the signals are emitted and received by the optical fiber's optical core, though for shorthand convenience, signal transmission is sometimes said to be by “the optical fiber.”
A desirable optical/optoelectronic coupling system will have high signal-to-noise ratio and high speed; low parasitic capacitance and inductance; high density; low crosstalk between devices; low power consumption; and the ability to integrate multiple functions. The resulting system will then have low cost and high performance.
A desirable optical/optoelectronic coupling system will have a reduced number of tightly specified or demanding manufacturing steps; will have fewer failure-susceptible parts such as wire bonds; and will generate a smaller amount of waste heat. Such devices must also meet multiple standards, such as a 250 μm pitch between each fiber; physical dimensions on the optical connections; and reliability characteristics including thermal cycling, humidity resistance and mechanical durability. An example of an optical fiber connector which is standardized is the “MT” ferrule, which precisely positions an array of optical fibers in a V-groove substrate and provides alignment means in the form of guide holes whose position relative to the fibers is tightly specified. The alignment means are used for alignment using complementary alignment members in the form of guide pins.
In some applications, space constraints dictate that the optical fibers must couple with optoelectronic devices whose primary direction of light access (light emission or reception) is at an angle to the direction that light is transmitted in an optical fiber core, e.g., typically approximately 90°. For example, in telecommunications networks, employing large numbers of lasers and detectors coupling to large numbers of optical fibers, there may be a limited amount of space between circuit boards arranged in stacks or there may be a strong incentive to have all coupling structures present a low profile to avoid impeding air flow that is desired to cool heat-producing devices. Further, if the optical fibers can be brought in along a mounting substrate (e.g., printed circuit board) on which the optoelectronic devices are mounted, to allow the optoelectronic devices to be situated as close as possible to other integrated circuitry, the distances that electrical signals must travel can be minimized. This consideration is especially important for maintaining electrical fidelity of the signal at high transmission rates, e.g. above a Gighertz. A desirable optical/optoelectronic coupling system will also provide efficient optical coupling between an optoelectronic device and an optical fiber in which the direction of light transmission in the optical fiber core is at an angle to the primary direction of light emission or absorption by the optoelectronic device.
A desirable optical/optoelectronic coupling system will also have the capability of enhancing transmission of light from the coupled optoelectronic device into the core of the optical fiber by permitting an optional light focusing step to take place prior to entry into the fiber. It should be noted that optical fibers are cylindrical, and hence if the surface of the fiber is allowed to act as a lens, it will be a spherical lens. At the size of an optical fiber, spherical aberration and chromatic aberration of spherical lenses are very disadvantageous. A preferred design approach will allow the inclusion of lenses designed to optimize optical coupling. Use of parabolic lenses and complex lenses offers the opportunity to reduce chromatic and spherical aberration. Lenses can also be designed which reduce alignment tolerances, thereby improving performance and lowering manufacturing costs.
A desirable optical/optoelectronic coupling system will be capable of processing optical signals between arrays of optoelectronic devices and arrays of optical fibers. Optical fibers are often provided in 1×12 arrays, although much larger one and two dimension fiber arrays are in development. The array sizes can be expected to grow as networks become more complex. Hence a useful coupling system will be configured to handle arrays in a simple and scalable fashion. Networks increase in both speed and number of connections in response to increasing data bandwidth demand, driven primarily by the growth of the Internet. As the network increases in size, the number of nodes increases exponentially, due to the requirements to make connections between multiple systems; to provide backup and storage functions; to provide redundant paths, and to allow flexible system operation.
The optical fiber arrays with which the optoelectronic devices must couple may advantageously be mounted in carriers such as “V-groove” substrates which have spaced-apart fiber-retaining V-grooves in their top surface. For the purposes of this specification, in relation to its associated optical fiber, a V-groove formed in a carrier substrate is a groove having two inwardly inclined planar surfaces for fiber alignment against each of which the curved surface of the associated optical fiber is able to make simultaneous line contact. The V-groove may be defined in its entirety by these planar fiber alignment surfaces meeting in a line at the base of the groove, or it may take a truncated form in which the two planar fiber alignment surfaces are connected by a third surface. Such V-grooves, truncated or otherwise, are grooves formed to provide precisely located and oriented planar fiber alignment surfaces using photolithographic techniques. Groove formation techniques may involve anisotropic etching of a crystalline substrate such as a single crystal silicon wafer (see, e.g., Bean, K. E., IEEE Transactions on Electron Devices, vol. ED-25, No. 10, October 1978, “Anisotropic Etching of Silicon,” pp. 1185-1193); or it may involve injection molding of plastic components, which is well known in the semiconductor packaging industry.
U.S. Pat. No. 4,897,711, “Subassembly for Optoelectronic Devices,” granted Jan. 30, 1990, describes methods of forming arrays of optical fibers mounted in V-groove substrates. Such v-groove optical fiber arrays have become well-known and accepted in optical network applications. Injection molded plastic components for mounting optical fibers, including V-groove substrates, are also well known, as for example those provided by Nissin Kasei Co., Ltd. (11-5 Senju-Kawaharacho, Adachi-ku, Tokyo 120-0037, Japan).
U.S. Pat. No. 4,730,198, granted Mar. 8, 1988, describes linear arrays of optical fibers aligned to linear arrays of optoelectronic devices on an elongated chip. The subassembly includes optical fiber segments embedded between V-groove mounting blocks and has the optoelectronic device array chip affixed to an end face of the blocks. The optoelectronic device array chip is therefore mounted orthogonally to the optical fiber blocks, and wire bonds are needed to provide an electrical connection to each optoelectronic device. Because the fiber mounting blocks are relatively thin, i.e. in the range of about 30 mils thick, the requirements of the electrical connection and the orthogonally mounted optoelectronic chip lead to a height mismatch and the need for a supporting member, which leads to added size, and with the wire bonds, added failure modes.
The anisotropic etching properties of silicon are also useful for aligning and mounting the optoelectronic devices. U.S. Pat. No. 4,897,711 describes packaging subassemblies for optoelectronic devices that align an optoelectronic device with various lenses, reflectors, and optical fiber transmission media. The supports for various optoelectronic devices and the reflecting surfaces are defined in monocrystalline silicon by taking advantage of known anisotropic etching properties of silicon. Because silicon etches preferentially along predictable crystallographic planes, various grooves, cavities and alignment detents can be quickly and easily made with great precision by masking and etching various surfaces of monocrystalline silicon support structures. The manufacturability of such optical/optoelectronic integrated modules is challenging, however, because of the need to etch very different profiles into the same silicon substrate, requiring many masking/demasking steps.
U.S. Pat. No. 5,243,671, granted Sep. 7, 1993, describes a method for coupling light from an edge-emitting laser chip into an optical fiber mounted in a substrate carrier having a V-groove etched therein. The fiber has a beveled end and is positioned in the plane of the laser chip (e.g., by being mounted on the same substrate) within the V-groove such that light emitted by the laser chip strikes the inner face of the beveled end and is totally internally reflected into the fiber core. The V-groove must provide an accurate vertical dimension so that the height of the fiber core can be precisely aligned with the laser output.
As was mentioned above, to accomplish the goal of an efficient optoelectronic/optical coupling method, it is first necessary to find methods to combine high density, high speed CMOS controlling circuitry with the associated optoelectronic devices in an intimate fashion in order to minimize parasitic capacitance and inductance and to increase density of optical interconnects. Various methods have been tried, with less than ideal results. Heteroepitaxial growth of GaAs on silicon, and silicon on GaAs have been explored, but after decades of research, fundamental problems such as the mismatch in the crystal lattice constants, cross-contamination, incompatibilities of device processing, and the difference in the coefficients of thermal expansion of the two materials have prevented this goal from being satisfactorily achieved, particularly when high performance lasers, photodetectors or drive electronics are required. Epoxy casting, by which completely fabricated chips are mounted in a common epoxy cast and final metal deposited (multi-chip modules) has numerous problems, including high cost and poor parasitics, size, reliability and yield. Flip-chip bonding, by which a chip is flipped over and attached to a substrate or other chip by a solder joint by which two dissimilar chips are brought into intimate electrical and mechanical contact with each other has been used for combining low temperature infrared (IR) detector arrays with silicon readout circuitry. There are commercial machines that can perform this operation with great reliability and repeatability. In this example, the silicon substrate is transparent to the infrared wavelengths being detected by the IR detector array. However, for optical wavelengths less than approximately 1 μm, silicon is too opaque for use as a transparent substrate, so this technique cannot be used and hence the substrate needs to be removed by etching or other means to provide optical access to the OE devices.
Furthermore, infrared detectors are generally low speed systems, often operating at Kilohertz speeds. For such low speeds, the parasitic effects of the silicon substrate are virtually non-existent. Optical communications systems operate at Gigahertz rates, or a million times higher in frequency. These high speeds cause the silicon substrate parasitics to become significant in areas of power consumption, crosstalk, distortion, maximum frequency and noise. These electrical limitations of silicon as a substrate material are true for all wavelengths of light being used. Therefore, even if the silicon is transparent at a given wavelength, it is not an ideal substrate for high speed (multi-Gigahertz) systems.
There is, therefore, a need for an improved commercially-viable method for coupling optical fibers to optoelectronic devices.